There’s been a lot of work in carbon nanotubes of late. The work comes from a broad assortment of labs and pushes the nano boundry along a broad front.
Northwestern University has developed a method for making carbon nanotubes in commercial quantities while solving a quality control problem.
Current methods for synthesizing carbon nanotubes produce mixtures of tubes that differ in their diameter and twist. Variations in electronic properties arise from these structural differences, resulting in carbon nanotubes that are unsuitable for most proposed applications.
Now, a new method developed at Northwestern University for sorting single-walled carbon nanotubes promises to overcome this problem. The method works by exploiting subtle differences in the buoyant densities of carbon nanotubes as a function of their size and electronic behavior. The results will be published online Wednesday, Oct. 4, in the inaugural issue of the journal Nature Nanotechnology (October 2006).
The researchers also maintain that their work can be reproduced on an industrial scale.
“The technique is especially promising for commercial applications,” said Hersam, “because large-scale ultracentrifuges have already been developed and shown to be economically viable in the pharmaceutical industry. We anticipate that this precedent can be straightforwardly translated to the production of monodisperse carbon nanotubes.”
Single-walled carbon nanotubes are coated in soap-like molecules called surfactants, then spun at tens of thousands of rotations per minute in an ultracentrifuge. The resulting density gradient sorts the nanotubes according to diameter, twist and electronic structure. Credit: Zina Deretsky (adapted from Arnold et al.), NSF
In another interesting advance that solves for the carbon nanotube twisting problem MIT researchers have discovered
that certain molecules can attach themselves to metallic carbon nanotubes without interfering with the nanotubes’ exceptional ability to conduct electricity.
MIT researchers have discovered that certain molecules can attach themselves to metallic carbon nanotubes without interfering with the nanotubes’ exceptional ability to conduct electricity. At left, the high conductance state has two molecular orbitals, shown in green. Some molecules even let the nanotube switch between highly conductive, left, and poorly conductive (right, with one red molecular orbital), creating the potential for new applications. Image courtesy / Marzari Lab
The work is reported in the Sept. 15 issue of Physical Review Letters.
Carbon nanotubes — cylindrical carbon molecules 50,000 times thinner than a human hair — have properties that make them potentially useful in nanotechnology, electronics, optics and reinforcing composite materials. With an internal bonding structure rivaling that of another well-known form of carbon, diamonds, carbon nanotubes are extraordinarily strong and can be highly efficient electrical conductors.
The problem is working with them. There is no reliable way to arrange the tubes into a circuit, partly because growing them can result in a randomly oriented mess resembling a bowl of spaghetti.
Researchers have attached to the side walls of the tiny tubes chemical molecules that work as “handles” that allow the tubes to be assembled and manipulated. But these molecular bonds also change the tubes’ structure and destroy their conductivity.
Now Young-Su Lee, an MIT graduate student in materials science and engineering, and Nicola Marzari, an associate professor in the same department, have identified a class of chemical molecules that preserve the metallic properties of carbon nanotubes and their near-perfect ability to conduct electricity with little resistance.
Using these molecules as handles, Marzari and Lee said, could overcome fabrication problems and lend the nanotubes new properties for a host of potential applications as detectors, sensors or components in novel optoelectronics.
Attaching a molecule to the sidewall of the tube serves a double purpose: It stops nanotubes from sticking so they can be processed and manipulated more easily, and it allows researchers to control and change the tubes’ electronic properties. Still, most such molecules also destroy the tubes’ conductance because they make the tube structurally more similar to a diamond, which is an insulator, rather than to graphite, a semi-metal.
Some molecular handles can even transform between a bond-broken and a bond-intact state, allowing the nanotubes to act like switches that can be turned on or off in the presence of certain substances or with a laser beam. “This direct control of conductance may lead to novel strategies for the manipulation and assembly of nanotubes in metallic interconnects, or to sensing or imaging devices that respond in real-time to optical or chemical stimuli,” Marzari said.
This approach might help appropriately charge carbon nanotube semi permiable membranes as well for desalination purposes.
Several weeks ago just the most interesting piece of science was done by University of Pittsburgh R.K. Mellon Professor of Chemistry and Physics John T. Yates Jr. I have suggested that the fastest way to get carbon nanotubes to sort out Na and Cl is to dope the nanotubes with impurities that charge the nantubes and therefor variously repel Na or Cl. Professor Yates shows water doing something entirely different.
In collaboration with J. Karl Johnson, who is the William Kepler Whiteford Professor of Chemical Engineering at Pitt, Yates has extensively investigated the use of single-walled carbon nanotubes (SWNTs) as tiny test tubes. SWNTs are cylindrical molecules with a diameter equivalent to about three atoms. The tube walls are made of a single curved sheet of carbon atoms.
Yates and Johnson, along with their students and postdoctoral fellows, obtained a striking result for water molecules confined inside SWNTs, as reported in a recent paper in the Journal of the American Chemical Society. The water molecules inside nanotubes bond together into rings made of seven water molecules. Yates and Johnson, who also are researchers in Pitt’s Gertrude E. and John M. Petersen Institute of NanoScience and Engineering, found that these rings stack like donuts along the nanotube. The rings themselves are bound together by a new type of hydrogen bond that is highly strained compared to the hydrogen bonds within each molecular “donut.”
The researchers first detected this novel hydrogen bond experimentally by its unusual singular vibrational frequency and later deduced its character by modeling. “The behavior of water as a solvent inside of nanotubes will probably differ strongly from its behavior in ordinary water based on the donut configuration and the new kind of hydrogen bond discovered in this work,” says Yates.
In another development, research showed that reactive molecules confined inside nanotubes are well shielded by the nanotube walls from reacting with active chemical species like atomic hydrogen, one of the most aggressive chemical reactants in the chemist’s toolbox. The work suggests that chemists could keep certain molecules from reacting by storing them inside nanotubes, while molecules outside the tube are free to react. “This could provide a new tool for focusing reactive chemistry in the laboratory to select one molecule and exclude another one, tucked away inside of a nanotube,” Yates says.
Yates’ approach to water and carbon nanotubes is pretty neat imho. Someone from the desalination research community needs to approach professor Yates and make him an offer he can’t refuse.
Navier-Stokes Equation Progress?
Penny Smith, a mathematician at Lehigh University, has posted a paper on the arXiv that purports to solve one of the Clay Foundation Millenium problems, the one about the Navier-Stokes Equation. The paper is here, and Christina Sormani has set up a web-page giving some background and exposition of Smith’s work.
Wikipedia describes Navier-Stokes Equations this way:
They are one of the most useful sets of equations because they describe the physics of a large number of phenomena of academic and economic interest. They are used to model weather, ocean currents, water flow in a pipe, motion of stars inside a galaxy, and flow around an airfoil (wing). They are also used in the design of aircraft and cars, the study of blood flow, the design of power stations, the analysis of the effects of pollution, etc. Coupled with Maxwell’s equations they can be used to model and study magnetohydrodynamics.
When these equations pass peer review they’ll be very helpful in algorithms that model fluids in a pipe or maybe even in a carbon nano tube.
One other thing. The Millenium Prize, worth $1 million is working well to advance scientific research.